1. Field of the Invention
The present invention generally relates to a semiconductor device. More particularly, the present invention is concerned with a semiconductor device which can be advantageously and profitably applied to a single-mode oscillation semiconductor device typified by a wavelength-tunable semiconductor laser device for a coherent optical communication, a semiconductor laser device for a high-speed communication and others.
2. Description of the Related Art
For a large capacity optical communication system the use of a wavelength-tunable semiconductor laser device and a modulating semiconductor laser device is required. For a better understanding of the present invention, description will first be made in some detail of the technology known heretofore in conjunction with these laser devices.
First, the wavelength-tunable semiconductor laser is considered. As one of the large-capacity communication systems, there can be mentioned a coherent optical communication system in which an interference receiver is employed not only for making it possible to increase the reception sensitivity but also for allowing only light signals of different wavelengths to be selectively extracted by varying the wavelength of the interference light for reception. In other words, with the coherent optical communication system, there can be realized a wavelength-multiplexed communication in which channel selection can be effected with the aid of the light wavelength by employing a laser light source having a changeable oscillation wavelength in the receiver. Intrinsically, the optical communication has a significantly increased transmission capacity because of the capability of high-speed modulation as compared with the electronic signal transmissions utilized heretofore. Further, in the case of the wavelength-multiplexed communication, it is possible to increase surprisingly the transmission capacity because a great number of light signals of different wavelengths can be transmitted through the medium of only one optical fiber. For these reasons, the coherent wavelength-multiplexed optical communication is now attracting increasingly public attention as one of the communication techniques for supporting the age of large-capacity communication anticipated in the not too distant future.
In order to realize the wavelength-multiplexed optical communication, the receiver side a semiconductor laser is required to have as broad a wavelength-tunable range or width as possible and capable of a single wavelength oscillation (single-mode oscillation).
As the wavelength-tunable semiconductor laser reported in the past, there can be mentioned a laser described in "Electronics Letters", Vol. 23, No. 8, (1987), pp. 403-405, the structure of which is shown in FIG. 8 of the accompanying drawings. This semiconductor laser is referred to as the DBR laser (an abbreviation of distributed Bragg reflection laser) and composed of three regions provided on a substrate 806, i.e. a distributed Bragg reflection region (DBR region) 801, a phase control region 802 and an optical amplification region 803, wherein p-type electrodes 821, 822 and 823 are provided independently on the above-mentioned regions, respectively, with a common n-type electrode 824 being provided on the lower surface of the substrate 806. An active layer 815 is formed solely in the optical amplification region 803, whereby light is amplified in the optical amplification region by carriers injected through the electrode 823. The two other regions include no active layer and are constituted with a passive optical waveguide 813 whose refractive index is forced to vary by the carriers injected through the electrodes 821 and 822.
As is known in the art, the oscillation wavelength of the DBR laser is determined by the Bragg reflection wavelength which in turn is determined on one hand by a product (optical pitch) of the effective refractive index experienced by the light transferred through the optical waveguide within the DBR region 801 and the pitch of a diffraction grating 812 implemented in the Bragg reflection region 801 and on the other hand by a resonance wavelength satisfying the phase condition of light traveling through the optical waveguide 813 reciprocatively between an edge 820 located on the side of the optical amplifier region and the DBR region. Consequently, in order to vary the laser oscillation wavelength continuously, it is necessary to vary simultaneously both the Bragg reflection wavelength and the resonance wavelength while maintaining both of these wavelengths so as to coincide with each other. According to the above-mentioned prior art technology, a continuous wavelength-tunable width or range is realized for a single wavelength (in a single-oscillation mode) by controlling both the Bragg reflection wavelength and the resonance wavelength.
In this conjunction, another example of the DBR laser is reported in "Applied Physics Letter", Vol. 52, No. 16, (1988), pp. 1285-1287. In the case of this known wavelength-tunable semiconductor laser, the DBR region is formed of an optically active material similar to the optical amplification region, wherein independent electrodes are provided separately for the purpose of allowing the oscillation wavelength to change under the influence of the change in the density of injected carriers, as in the case of the preceding example. However, the DBR laser now under consideration differs from that disclosed in the first mentioned literature in that a greater change of the refractive index ascribable to an absorption edge shift effect which accompanies the carrier injection is made available by using the optically active material for the DBR region. By virtue of this structure, a value as large as 11.6 nm is realized for the wavelength-tunable width. (It is however noted that the tunable width or range realized in this prior art DBR laser is not for the continuous change of wavelength but for discrete change thereof.)
The structure of the second mentioned DBR laser however suffers from the problems in that absorption loss brought about by the free carriers is increased as the amount of carrier injected increases, because a passive material is used for the DBR region and the phase control region, whereby the value or level of threshold current for oscillation of the optical amplification region is increased, which results in lowering of the laser output power and increasing in the spectral line width. In this conjunction, it is to be noted that the coherent optical communication requires a narrow spectral line width of the laser light. Accordingly, the phenomenon of increasing the spectral line width by changing the wavelength can never be tolerated.
A semiconductor laser device developed in an effort to solve the technical problem of the increasing of the spectral line width is disclosed, for example, in JP-A-64-49293 (Japanese Patent Application Laid-Open No. 49293/1989). In this known semiconductor laser, an optically active layer exhibiting a gain is provided in the DBR region. With this known semiconductor laser structure, it is certainly possible to compensate for the absorption loss occurring upon current injection into the phase control region by the gain of the active layer provided in the DBR region, whereby the spectal line width can be suppressed from increasing. Further, such a structure is also known in which the phase control region and the optical amplification region are finely divided and arranged alternately with each other for thereby protecting the laser characteristics against degradation due to the absorption loss mentioned above. In this conjunction, reference may also have to be made to JP-A-64-14988.
Now, description will be turned to the semiconductor laser for modulation.
In the optical communication system, one of the requisite performances imposed on the system is how densely the signal can be transmitted and received. To this end, high-speed response capability must be ensured in both the light signal transmitter and receiver devices. In general, modulation of the semiconductor laser light with a high-speed current pulse signal is ordinarily attended with fluctuation in the wavelength due to change in the refractive index internally of the laser. This phenomenon is known as wavelength chirping. Causes for wavelength chirping will be explained below.
The density of carriers injected in the active region of the semiconductor laser by current pulses of a modulation signal is forced to change in accordance with the modulating signal. In this conjunction, it is noted that a phase delay occurs in the laser output light pulse relative to the change in the carrier density. As the consequence, the carrier density becomes dominant such that the carriers exist in excess when compared with the stable state.
Upon oscillation of the laser light, the carrier density tends to decrease toward the stable state as a result of the stimulated emission. This means that during the oscillation of the laser light, the carrier density continues to change. Since the refractive index of the active layer has a dependency on the carrier density, the change thereof brings about corresponding fluctuation in the laser oscillation wavelength. This is a cause of the wavelength chirping phenomenon. Since the optical fiber employed for the optical communication or the like application exhibits a wavelength-dependent dispersion of the refractive index (wavelength dispersion), occurrence of the wavelength chirping gives rise to distortion in the pulse waveform. This provides a major factor for limiting the distance of transmission in the high-speed transmission.
With a view to reducing or suppressing the wavelength chirping taking place upon high-speed modulation of the semiconductor laser, there has already been reported a method of applying electric currents to a plurality of electrodes disposed in cascade along the direction of the resonator. In this conjunction, reference may have to be made to "IEEE Journal of Lightwave Technology", LT-5, No. 4, (1987), pp. 516-522. More specifically, according to this known method, a plurality of electrodes are provided for the active regions having a same composition (same bandgap), wherein the mutually different electric currents are so applied to the plural electrodes that the wavelength chirping can be reduced.